1. Field of the Invention
This invention relates to the synthesis of nanostructured materials by chemical methods. In particular, this invention relates to the synthesis of nanostructured oxides and hydroxides via chemical methods, thereby allowing the production of materials of controlled morphology, phases, and microstructures, including a new class of fibrous microstructures which combines a high density of chemically active sites with enhanced fluid percolation rates.
2. Description of the Related Art
Materials having fine-scale microstructures possess unique and technologically attractive properties, as shown by work on rapidly solidified metals, alloys and composite materials wherein grain sizes on the order of a few micrometers (microns) are obtainable. Recently, however, research has focused on reducing grain size from the micrometer to the nanometer range. A feature of such nanostructured materials is the high fraction of atoms (up to 50%) residing at grain or particle boundaries. FIG. 1 illustrates this phenomena schematically, wherein white circles depict grain boundary atoms and solid circles depict interior atoms. The high fraction of the atoms residing in surface boundaries is important in creating a high density of potential sites for catalytic and electrochemical reactions. Nanostructured materials, which refer herein to materials having a grain diameter of about 1 to 100 nanometers (1 nm=10 angstroms) possess substantially different, and in many cases improved, chemical and physical properties compared to their micron-sized grain counterparts of the same chemical composition.
Nanostructured powders have been previously synthesized by chemical methods from aqueous solution. A typical prior art synthesis procedure involves three sequential steps: (1) preparation of an aqueous starting solution of mixed metal salts, (2) reductive decomposition of the starting solution to obtain a colloidal suspension of the desired end-product phase, and (3) separation of the end-product powder by repeated washing and drying. The resulting dried powder products are in the form of loosely agglomerated nanoparticles. Metal chloride solutions can be reduced with sodium trialkylborohydride to form nanostructured powders of nickel or iron, and mixed metal chloride solutions can be reduced to form nanostructured powders of M50 steel, AIN/BN, NiCr/Cr.sub.3 C.sub.2, and WC/Co.
The synthesis of oxides and hydroxides in nanostructured form facilitates the manufacture of components and devices with modified and/or superior performance. An additional benefit of grain size refinement to the nanoscale range is the elimination of large voids at grain boundaries, which often contribute to deleterious properties. The recent demonstration that nanostructured ceramic materials (i.e., TiO) can be superplastically deformed at relative low temperatures and pressures, as described in the examples in Nanostructured Materials, Vol. 1, 1992, has important implications in net shape forming brittle ceramic and intermetallic components. In industrial coating applications, for example thermal barrier coatings, reduction to the nanoscale is highly effective in increasing thermal impedance. In dense ceramic coatings, nanostructured materials have the potential to provide high hardness combined with good fracture toughness, and corrosion resistance.
High surface-area materials with nanoscale dimensions are of special interest in applications where active-site mediated chemical reactions play a decisive role. In catalytic applications, a high contact area for oxidation and reduction with the surrounding environment is important, and thus reduction of the catalytic material to the nanoscale is clearly a distinct advantage. Catalytic applications include pollution control, such as nuclear waste mitigation, water purification, mercury remediation, particulates remediation, and air filtration, as well as catalysis for synthetic purposes, such as molecular sieves, petroleum refinement, and the like. However, despite a strong interest in the development of nanostructured materials for catalytic applications, the present nanostructured materials suffer from the tendency of the particles to form agglomerates in which the interparticle pore space becomes comparable with the particle size, that is, wherein the interparticle pore space itself has nanoscale dimensions. These small pore sizes limit the percolate rate of the active species into and through the agglomerates.
Another area for the use nanostructured materials is rechargeable batteries and fuel cells, where the high surface area of the nanostructured materials promotes rapid interactions of the active material with the surrounding media. In high energy density rechargeable storage batteries, for example, the need to sustain high current pulses under charging and discharging conditions requires maximum contact between the electrode and electrolyte to achieve a high density of ion and electron carriers. Active nanostructured materials, with their high density of controlled surface defects, address this requirement, thus providing the means to optimize the high energy storage capacity of batteries.
The nickel electrode in particular has a ubiquitous and critical presence in rechargeable batteries, as it is generally the capacity-limiting electrode in Ni-Cd, Ni-Zn, Ni-H.sub.2 and Ni-MH cells. Previous studies have shown nickel hydroxide (Ni(OH).sub.2) can be formed by cathodic deposition from solution, and by precipitation from concentrated alkali. Traditional nickel electrodes are fabricated from a porous nickel plaque made from nickel carbonyl powder, Ni(CO).sub.4. Porosity is typically limited to 80%, and the volume take-up by the sheet and plaque is near 20%. This plaque is then either chemically or electrochemically impregnated with active material. Japanese workers have pioneered the development of a high-performance spherical nickel hydroxide (Japanese Tanaka) for application in foam or sintered fiber mat nickel electrodes. These substrates are highly porous (about 95%) so that a large volume of active material can be loaded into the electrodes. This represents a radical departure from the traditional use of sintered carbonyl nickel-type electrodes that require complex manufacturing processes to either chemically or electrochemically deposit the active material within the porosity of the plaque.
Nickel hydroxide materials have not yet been synthesized in a nanostructured form. In current practice, micron-sized nickel hydroxide is synthesized by chemical precipitation and electrolytic deposition. Work on micron-scale nickel hydroxide-based materials indicates three forms of crystal structure, namely the hexagonal .alpha.- and .beta.-phases and the cubic .gamma.-phase. In nickel electrode applications, the .beta.-phase is usually used because of its stability during charge-discharge cycle process. However, a-nickel hydroxide, although unstable in the charge-discharge cycle, is capable of storing a higher amount of energy due to a higher valence charge. Current nickel electrodes are less than ideal because of the low volumetric energy density of the active materials. The theoretical x-ray density of nickel hydroxide is 4.15 g/cc, but the present electrode can only achieve a density of 1.8 g/cc. This is primarily due to the large micro-sized voids associated with the processed electrodes when using conventional nickel hydroxide.
Manganese dioxide (MnO.sub.2) is also not presently available in a nanostructured form. The particle size in both naturally occurring and commercially synthesized manganese oxide is on the micron, or even millimeter scale. Naturally occurring manganese dioxide is extremely impure, with a number of oxide impurities, such as SiO.sub.2, Fe2O.sub.3, Al.sub.2 O.sub.3, and P.sub.2 O.sub.5. These impurities complicate the chemical and structural analysis of naturally-occurring manganese dioxide, and limits its range of applications.
Much interest has therefore been focused on methods for the synthesis of manganese dioxide, including ion exchange, hydrothermal synthesis, electrolytic synthesis, and chemical synthesis. Chemical methods developed in the 1970's yield pure, micron-sized manganese dioxide in a variety of crystalline forms. Subsequent syntheses by the reaction of manganese salts (MnCl.sub.2 or MnSO.sub.4) with a strong oxidizer (KMnO.sub.4 or a mixture of ozone and oxygen) results in layered manganese dioxide. However, little or no attempt has been made to produce manganese dioxide materials on the nanostructured scale or with a controlled morphological form.
Crystallographic studies reveal that at the molecular level manganese dioxide is constructed from MnO.sub.6 octahedrons, each consisting of six oxygen atoms surrounding a manganese atom. The octahedrons are linked at their vertices and edges to form single or double chains. These chains share corners with other chains, leading to structures with tunnels or channels formed by rows of atomic empty sites. The size of these channels is related to the number of manganese-oxygen chains on each side. The presence of channels facilitates the transport of mobile ionic species, including Li.sup.+, H.sup.+, K.sup.+, Ba+.sup.2, Na.sup.+, or Pb.sup.+2. This feature is important because such cation exchange enhances both catalytic properties for oxidation reactions, and good ionic/electronic conduction for energy storage batteries.
Zirconia (ZrO.sub.2) is another oxide of particular interest because of its chemical stability, high hardness, refractory nature (ability to withstand high temperatures), and ionic conductivity. Structurally stabilized zirconia is widely used in thermal barrier coatings for advanced engines, which are subject to extremely high temperatures. Other uses of zirconia include milling balls, refractors, oxygen sensors, and fuel cells batteries, as well as electronic ceramics.
Zirconia has a monoclinic structure at low temperatures, but exists in different forms at elevated temperatures. For example, undoped zirconia with the monoclinic structure transforms near 1170.degree. C. to the tetragonal structure, and then near 2370.degree. C. to the cubic structure. This transformation is accompanied by a volume change, which can lead to mechanical damage of parts. The presence of lower-valance cations such as Mg.sup.+2, Ca.sup.+2, Y.sup.+3, and rare earth cations stabilizes the high temperature phases to lower temperatures so that metastable tetragonal or cubic phases can exist down to ambient temperature.
Methods used to produce conventional micro-scale stabilized ZrO.sub.2 includes co-precipitation, microemulsion, and sol-gel synthesis. Current techniques for synthesizing nanostructured zirconia and yttria-stabilized zirconia (Y.sub.2 O.sub.3 /ZrO.sub.2) are not economic as commercial processes. The inert gas condensation (IGC) and chemical vapor condensation (CVC) methods are inherently slow and thus not cost effective, while nanoparticles produced by sol-gel synthesis are heavily agglomerated.
As can be seen by the above discussion, despite recent developments in the synthesis of nanostructured materials, there still remains a need for materials and methods which are economical, and which produce materials which are suitable for a wide range of industrial applications. There remains a particular need for materials suitable for catalytic applications, that is, materials having a high density of active sites, and yet good percolation rates.